1. Field of the Invention
The present invention relates to a communication terminal, a congestion control method, and a congestion control program, and more specifically to a congestion control method in a TCP (transmission control protocol).
2. Description of the Related Art
Recently, in the most widely used TCP in the Internet, congestion control is performed using a congestion window (CWND) when a segment is transmitted to avoid the congestion of a network.
Generally, the congestion control algorithm of a TCP sets a congestion window as described below when a packet loss occurs.
1. At FF (fast retransmit & fast recovery), the following values are set.SSTHRESH=(1−b)×CWND CWND=SSTHRESH2. When RTO (retransmission timeout) occurs, the following values are set.SSTHRESH=(1−b)×CWND CWND=1×MSS 
However, an MSS (maximum segment size) is the maximum segment size that can be transmitted on the transmitting side. A CWND (congestion window) is a TCP state variable for restriction of the permitted transmission size of a TCP. The SSTHRESH (SLOWSTART THRESHOLD) is a slow start threshold.
In a normal TCP, a=1 and b=0.5 are used.
In addition, for example, the non-patent document 1 discloses a method of effectively utilizing network resources after an occurrence of a packet loss by calculating the values of the CWND and the SSTHRESH while estimating the bandwidth in the bottleneck link based on the reception intervals of the Ack (acknowledgment) when a packet loss is detected.
That is, when a packet loss is detected, the values of the CWND and the SSTHRESH are calculated as follows.bk=dk/(tk−tk−1)  (1)
However, tk indicates the time when the k-th Ack is received, and dk indicates the number of bytes of segments for which the receiving side acknowledges the reception.
Then, the moving average of keyboard in the equation (1) is calculated by the equation (2), and bk in the equation (2) is used as is as the values of the CWND and the THRESH.bk=19/21·bk−1+2/21 ·(bk−bk−1)/2  (2)[Non-patent Document 1] L. A. Grieco and S. Mascolo: End-to-End bandwidth Estimation for Congestion Control in Packet Networks, Prc. Quality of Service in Multiservice IP Networks (QoS-IP 2003. (Milano), February 2003.